A very common scenario one sees online, and occasionally sees in fiction is a post apocalyptic invasion by China on our west coast. The presumption is that after some sort of EMP strike has laid us low, the Chinese simply need to march on in to the undefended territory: it will all be there for the taking.
We will ignore for the moment that historically the Chinese have been extremely uninterested far-expansion and that they would have numerous closer at hand targets of interest. We will also ignore how they are going to transport all these troops.
Fortunately in a very rough sort of way, California is the same size as Iraq. So some of the numbers that I have previously looked at to determine if a U.S. war with Iran ( via Iraq) was a good idea (it was not) .
A typical Chinese division is going to require 100 to 400 pounds per person per day of supplies. This is a very low number based on what the Russians use and dropping it a little. In Iraq we generally had around 180,000 troops stationed. So let’s use that number as a conservative low estimate of what would be required of the Chinese to hold the post-apocalyptic major centers on the west coast. With 180,000 troops in California averaging 100 pounds per days if they are not doing much and 250 pounds per day if they are (non-intense) fighting with the local insurgents. That is 18,000,000 (9000 tons) to 45,000,000 pounds (22,500 tons) PER DAY. This does not include feeding the local populous, but also presumes that they will not be able to live off the land either: in general modern armies have little ability to live of the land.
Many of the scenarios have the Chinese taking Alaska and coming down the coast of Canada. It is an odd scenario that must look good on a map. It looked good on a map to the Japanese, and the United States worried about it during WW2, so let us explore it briefly.
There is only one continuous hardened highway line between Alaska and California The Alaskan Highway. It was built over some very rough terrain by the United States (with Canadian permission) during World War 2. It runs about 1,390 miles. There are a number of tie-ins to this road, but none appear to offer an independent routing to the west coast with Watson Lake in the Canadian Yukon being a major choke point.
A hardened road can handle about 20,000 tons of supply a day. A number that would be close to adequate for non-intensive combat needs. The carrying capacity of a military truck is usually from 3 to 10 tons. Civilian trucks (tractor trailers really) can probably get up into the 40 ton range with a piggy-back but I seriously doubt they would be available to the Chinese. Fuel Tanker trucks are probably going to range from 12 to 27 tons of fuel. Each truck at 60 miles per hours (and two drivers) can just barely make a one way trip to reach the West Coast. Then the West Coast itself is almost 1,300 miles long. So if the typical truck runs in and out of Alaska with 9 tons of stuff on it, you need about 3,000-7,500 truck runs a day to get to the United States and back with an average 3 day trip per truck. That is a lot of trucking.
So if the U.S. Air Force is gone, why not fly it in. Well at the very height of the Iran-Afghanistan conflict the United States at great cost was able to airlift in about 4 days of supplies every month by flying supplies into Turkey and then making 3,300 convoy truck deliveries per month: about 990 tons per day or 1/10th of non-combat minimum. It is highly unlikely the Chinese would do better.
The Chinese don’t have a great merchant marine, but neither does the United States any more. Presume that, as the United States did for Iraq, they will use contract shipping. Presumably there are no US nuclear submarines around as it is unclear what the response of the contractor will be to the risk of having their ships sunk. Modern container ships are very large, they are very often in the 4,500 -5000tea ( a container unit sizing. A typical container cargo is 10,000 to 12,000 long tons (2,240 pounds). So the Chinese army "only" requires about 10 to 25 of these very large ships a month to arrive at one of the five west coast container ports: Los Angeles, Port of Long Beach, Port of Oakland, Port of Seattle, Port of Tacoma. Since modern container ships are fast (20 knots/hour) that is about 14 days from Shanghai to California. We only need one set of ships. Since there are currently 4,831 container ships in the world, and 13,175 tankers, we can presume that the Chinese can scrape them up.
With the trucking runs being much shorter, and presuming a truck can make one round trip per day, we are now down to 1,000- 2,500 truck runs/day. With multiple delivery routes, there is no stress on the road network. Much more manageable.
Of course we have a problem. Figures from 2007 show the entire Chinese Army using about 1,078,000 tons of fuel per year (2,954 tons/day). Typically 60% of the weight in supplies is fuel. So if we take our lowest figure that would be 5,400 tons of fuel per day without even including all those truck runs to get them there supplies. The high number would be 13,500 tons of fuel per day. It also does not include fuel required for their air force, or supplies to base their air force within the United States, or their navy off our coast. With Chinese Civilian consumption set at 330,000 tons/day the Chinese would begin running out of their 7 million tons of oil reserves in less than a month.
Does this mean it would be impossible? No it does not. But the Chinese are going to need to get their hands on a lot of oil first. Presumably they could buy it, but how readily available is that much oil going to be in a post-apocalyptic world. And if the Chinese acquire enough oil to run an invasion of our west coast in a post apocalyptic world: why bother? They have the oil; they have won. Why go to the trouble?